1. Field of the Invention
The present invention relates to a membrane-electrode assembly for a solid polymer electrolyte fuel cell that provides high durability in generating electricity at high temperatures and superior proton conductivity in the low humidity region.
2. Related Art
A fuel cell is a power generation device that causes an electrochemical reaction of hydrogen, which is obtained from hydrogen gas or by reforming various hydrocarbon fuels (natural gas, methane, etc.), and oxygen in air, whereby electricity is output directly. In addition, the fuel cells have been attracting attention as a pollution-free power generation system that can directly convert chemical energy of fuels into electric energy with high efficiency.
The fuel cells are constructed from a pair of electrodes (fuel and air electrodes) supporting a catalyst and a proton-conductive electrolyte membrane (hereinafter referred to as a “proton conductive membrane”) sandwiched between the electrodes. In the fuel cells, after hydrogen is separated into hydrogen ions and electrons by the catalyst of the fuel electrode, the hydrogen ions travel through the proton conductive membrane and reacts with oxygen at the air electrode thereby to form water.
In recent years, there has become a demand for fuel cells to have higher power generation performance. In order to enhance power generation performance, operating at higher temperatures during power generation is required. Therefore, a proton conductive membrane for use in fuel cells is sought having high proton conductivity under a wide range of environments, in particular under high temperature environments.
Polymers having a sulfonic acid group have been conventionally used for such a proton conductive membrane. The present inventors have also proposed a certain polymer having a sulfonic acid group as a proton conductive membrane with high proton conductivity (e.g., see Patent Documents 1 to 4).    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2004-345997    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-346163    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-346164    [Patent Document 4] International Publication WO 07/010,731 Pamphlet